1. Field of the Invention
This invention relates to an electrostatic image recording method wherein visible images are formed by developing electrostatic latent images on the surface of an insulator by an electrophotographic process.
More specifically, this invention relates to a method of recording images by use of ionizing radiation such as X-ray, .alpha.-ray, .beta.-ray, .gamma.-ray, ultraviolet-ray or the like.
2. Description of the Prior Art
In the conventional electrographic printing process, electrostatic recording materials including a dielectric layer deposited on a conductive support for the purpose of image-recording, or containing a recording layer made of an insulator placed on a conductive interlayer which is additionally positioned on a dielectric support have been employed. An electrostatic latent image can be produced on such a recording material by allowing charged particles to adhere to the recording layer thereof according to the imagewise distribution.
Electrostatic latent images can be formed by various conventional methods. For example, there has been known a method which comprises scanning the surface of a dielectric layer with a needle impressed with voltage wherein the needle and the dielectric layer are put into contact or near contact as disclosed in Japanese Patent Publication No. 16453/69. Further, there is known a method which employs electric current created by photoelectric effect caused on a light-exposed area of a material placed in face-to-face relationship with a recording material as disclosed in U.S. Pat. No. 3,508,477. Another method comprises the steps of applying a potential between a photoconductor provided in contact with a dielectric surface and a recording material, and at the same time subjecting the photoconductor to imagewise exposure as described in U.S. Pat. No. 3,326,709 and by Eichi Inoue in "Electrophotographic Techniques" (Denshi Shashin Gijutsu published by Kyoritsu Publishers), particularly on page 43. Also is known a method which comprises applying charges directly on a recording material by irradiating it with an electron beam as disclosed in U.S. Pat. Nos. 2,200,741 and 2,281,638. Further, there is known another method which comprises erasing in an imagewise pattern charges uniformly provided on the surface of a recording material in accordance with the pattern.
In addition to the widely prevailing method using silver halide photographic materials, various methods for recording ionizing radiation images are known, such as xeroradiography as disclosed in, for example, U.S. Pat. No. 2,666,144. Xeroradiography, based on the principle of xerography, comprises forming an electrostatic latent image by use of X-rays instead of visible light, the latent image consisting of charged and uncharged areas corresponding respectively to non-irradiated areas and areas irradiated with X-rays, and then developing the image with a toner. If necessary, the toner image thus obtained can be transferred to a plastic sheet or plate, or the electrostatic latent image can be transferred to a resin plate before being developed whereby development is carried out on the resin plate.
Further, still other ionographic methods are known, one of which is called "ionography" as set forth in U.S. Pat. No. 2,900,515 and Japanese Patent Public Disclosure No. 82791/73, and another of which is reported in "Zeitschrift fuer Angewandte Physik", Vol. 19, p. 1 - 4 (Feb. 19, 1965). Both of these techniques are capable of providing increased sensitivity up to several ten times as high as that of xeroradiography. In the former process, gas molecules (mostly of large atomic number) sealed in a space are dissociated by the ionizing radiation. The dissociated charges are collected on an insulating film to produce an electrostatic latent image corresponding to the distribution of the radiation intensity, and then the latent image is converted to a visible image by applying a suitable developer material thereon. On the other hand, the latter process utilizes an arrangement comprising a cathode of heavy metal of large atomic number such as lead, and an anode of light metal of small atomic number such as aluminium placed parallel to each other across an intervening space filled with a gas. When ionizing radiation is directed thereon together with the application of an electric field, photoelectrons starting from the cathode advance through the gas layer towards the anode, and are accelerated by the electric field thereby to dissociate a number of gas molecules. The number of electrons is markedly increased and these electrons form an electrostatic latent image on an insulating layer provided on the anode. The electrostatic latent image thus prepared can be developed into a visible image, as in the former method, using any of the conventional electrophotographic development processes.
As examples of similar methods for forming an electrostatic latent image, there are further known a method of utilizing an electrical discharging phenomenon which takes place in gas molecules irradiated with X-rays as disclosed in U.S. Pat. No. 3,692,948, a method which takes advantage of the variation in ion current caused by a photoconductive mesh being irradiated with X-rays as disclosed in Japanese Patent Public Disclosure No. 98247/74, U.S. Pat. No. 3,603,790, Japanese Patent Public Disclosure No. 32534/73, etc., a method of multiplying ion current generated in gas molecules using micro-channel as disclosed in Japanese Patent Public Disclosure No. 17640/75, etc., a method of employing a liquid as a material capable of generating photoelectrons as disclosed in Japanese Patent Public Disclosure Nos. 137176/75, 87793/75 and 92733/75, and a method of erasing charges which have been uniformly provided on the surface of a recording layer by collecting the charges in an imagewise pattern by the above-described methods.
A feature common to these methods is that electrostatic latent images are produced by charged particles generated by ionizing radiations passing through a gas or a liquid layer and collected on the surface of an electrostatic recording layer.
On the other hand, it is known that the conventional developing method using a liquid developer is capable of converting the electrostatic latent image prepared by one of the abovedescribed electrostatic latent image forming methods or ionizing radiation image recording methods into a visible image of the highest quality known in prior art and of appreciably high image-density considering the low surface potential of the recording layer.
Nonetheless, a kind of image blur known as the "streak" phenomenon generally arises in the course of development, with a liquid developer, of the electrostatic latent image formed on the smooth surface of a recording layer such as is possessed by the common electrostatic recording materials. The streaks are caused by toner particles attaching to the areas of low density in the vicinity of an area of high density in a pattern resembling the tail of a comet. This phenomenon not only deforms the image, but also lowers the quality of the image by degrading the sharpness, resolving power and contrast. It further lowers the maximum density of the image.
In the ionizing radiation image recording methods wherein an electrostatic latent image is produced by charges passing through a gas layer or a liquid layer onto the surface of a recording layer of a recording material and adhering thereto in an imagewise distribution, there is another disadvantage in addition to the aforesaid streak phenomenon in that the electric charges first adhering to the surface of the recording layer raise the potential thus causing the repulsion of electric charges which arrive later in the course of charge collection. This repulsion phenomenon between the charges first adhering to the surface and the charges coming to adhere thereto later adversely affects the efficiency of collection of charges on the surface of the recording layer and not only causes a reduction in image density because of the smaller number of collected charges, but also degrades the sharpness, resolution and contrast of the image because the charges coming to adhere to the surface, mainly to the areas of high potential, are deflected in other directions in the gas layer so that the charges adhere to other areas where they should not adhere.